A Raster Output Scanner (ROS) and a Light Emitting Diode (LED) print bar, also known as imagers and used in electro-photographic machines are well known in the art. The ROS or the LED print bar is positioned in an optical scan system to write an image on the surface of a moving photoreceptor belt. The photoreceptor belt is then used to transfer the image onto a piece of paper. In some systems, the output of an ROS imaging station is used to write a black image onto the photoreceptor belt and the output of an LED print bar imaging station is used to add a color component to the image. Obviously, alignment of the two imaging stations is important for the reduction of blurring of the colorized image. Several factors affect the ability to accurately align the two imaging stations.
One such factor is that certain errors that are not present in an LED print bar imaging system are inherent to the physical structure and operation of an ROS imaging system. A typical ROS imaging system 10 as shown in FIG. 1 includes a light source 12 for generating a light beam 14 and a mirror 16 for directing the light beam 14 to a spot 18 on a photosensitive medium 20. The mirror 16 also serves to move the spot 18 to generate a scan line 22 of specified width along the photosensitive medium 20. To that end, the mirror 16, which in this embodiment comprises a rotatable polygonal mirror with a plurality of light reflecting facets 24 (only four of eight facets are shown), is rotated about a rotational axis 26 in the direction of an arrow 28. Thus, light impinging on the mirror 16 is swept across the photosensitive medium 20 as the mirror 16 rotates as described below.
The light source 12, which in this embodiment is a laser diode, emits a modulated coherent light beam 14 of a single wavelength. The light beam 14 is modulated in conformance with an image information data stream contained in the video signal sent from the light source control circuit 30 to the light source 12. The modulated light beam 14 is collimated by a collimating lens 32 and then focused by a cross-scan cylindrical lens 34 to form a spot on a reflective facet 24 of the rotating polygonal mirror 16.
As the polygonal mirror 16 is rotated around its axis of rotation 26 by a conventional motor (not shown), the beam 14 is reflected from the facet 24 and passes through the f-theta scan lenses 36 and the anamorphic wobble correction lens 38. The f-theta scan lens 36 consists of a negative plano-spherical lens 40, a positive plano-spherical lens 42, and the cross-scan cylinder lens 44. This configuration of f-theta scan lenses has sufficient negative distortion to produce a linear scan beam. Thus the light beam that is deflected at a constant angular velocity from the rotating mirror is theoretically modified optically by the f-theta scan lens to scan the photosensitive material 20 at a constant linear velocity.
After passing through the f-theta scan lens 36, the light beam 14 passes through the wobble correction anamorphic lens element 38. The wobble correction element can be a lens or a mirror and is sometimes referred to as the “motion compensating optics.” The purpose of the wobble correction element 38 is to correct wobble along the scan line 22 generated by inaccuracies in the polygonal mirror/motor assembly. The wobble correction element 38 then focuses the light beam in the cross-scan plane onto the scan line 22 on the photosensitive medium 20.
Accordingly, as the polygonal mirror 16 rotates, the light beam 14 is reflected by the facets 24 through the f-theta lens 36 and the wobble correction element 38 and scans across the surface of the photosensitive medium 20 along the scan line 22 from a first end 46 of the scan line 22 (referred to herein as the Start of Scan or “SOS”) past a center position (the illustrated position of the spot 18) and on to a second end 48 of the scan line 22 (referred to herein as the End of Scan or “EOS”). The light beam 14 thus exposes an electrostatic latent image on the photosensitive medium 20. As the polygonal mirror 16 rotates, the exposing light beam 14 is modulated by a pixel clock (not shown) in the light source control circuit 30 to produce individual bursts of light that expose a line of individual pixels, or spots 18, on the photosensitive member 20.
It should be noted that the path followed by the light generated by the imaging station 10 does not change from one scan to the next. Thus, the latent image on the photosensitive medium 20 is generated by rotating the photosensitive medium 20 past the imaging station such that on a subsequent scan, a scan line is produced on the photosensitive medium that is adjacent to the previously produced scan line.
Ideally, the ROS imaging system exposes a line of evenly spaced pixels 18 on the photosensitive medium 20. For example, FIG. 2 shows a scan line 50 consisting of a series of pixels 52 uniformly spaced at a distance 54 based upon timing pulses from the pixel clock of the ROS imaging system for the designed resolution (e.g., the central portion of each pixel position is evenly spaced at 1/300 inch intervals for 300 dpi resolution or is evenly spaced at 1/600 inch intervals for 600 dpi resolution, etc.). Also shown in FIG. 2 are a plurality of desired pixel location lines 56. The desired pixel location lines 56 may be thought of as a series of parallel lines perpendicular to the scan line 50. The points at which the scan line 50 intersects the desired pixel location lines 56 indicate the desired locations for pixel placement. Thus, in the idealized schematic of FIG. 2, each pixel 52 is aligned with a desired pixel location line 56 and each desired pixel location line 56 is spaced apart at an even distance 54.
As a result of the inherent geometry of the optical system of the ROS, and because manufacturing errors can cause imperfections in the facets of a polygonal mirror, obtaining evenly spaced, identical pixels can be problematic. The uneven spacing is referred to as “scan non-linearity.” FIG. 3 illustrates deviation from the uniform pixel placement of FIG. 2 due to scan non-linearity. The scan line 58 consists of a series of pixels 60 shown with the desired pixel location lines 62. The spacing of all of the pixels 60 does not coincide with the desired pixel location lines 62 as indicated by the scan line non-linearity error 64. The displacement of each of the pixels or pixel locations from the corresponding desired pixel location line may be shown by a curve referred to as a scan line non-linearity curve.
FIG. 4 shows a scan line non-linearity error curve 66 that reflects a zero position error at the SOS pixel 68 and the EOS pixel 70. The error curve 66 further includes both positive lobes 72 and negative lobes 74 between the SOS pixel 68 and the EOS pixel 70. Ideally, the error curve would be at zero across the entire scan line.
Scan line non-linearity is typically caused by system geometry or a velocity variation of the mirror 16. A scanner having a multifaceted rotating polygonal mirror, for example, directs the light beam toward the photosensitive material at a constant angular velocity; however, the photosensitive material 20 is farther from the polygonal facets 24 at the ends of the scan line 22 than it is at the center spot 18 as shown in FIG. 1. Accordingly, the speed at which the focused exposing light beam travels across the scan line 22 on the photosensitive medium 20, or spot velocity, is higher toward the ends of the scan line 22 and lower toward the center of the scan line 22. Without correcting for this inherent scan line non-linearity, the image being transferred to the photosensitive material will be distorted.
Distortion of the transferred image is small. Moreover, in an electro-photographic machine with only a single imaging station, such as a black and white imaging station, all of the images transferred to the photosensitive material include the identical distortion. Accordingly, the transferred image is generally not noticeably distorted even when no correction is made for the scan non-linearity. Such an imaging station in a machine that includes a second imaging station, so as to produce a colored image, however, results in noticeable distortion.
Noticeable distortion occurs as a result of the use of LED print bar imaging systems to provide the color image on the photosensitive material. LED print bar imaging stations generally consist of a linear array of light emitting diodes. As the photoreceptor material is advanced in the process direction, each LED in the LED print bar is individually controlled to produce a respective pixel. Therefore, because the pixels are not being created by a single centrally located source of light, the LED print bar imaging systems do not normally generate images with scan line non-linearity errors of the same magnitude as the scan line non-linearity errors generated by an ROS imaging system. Accordingly, overlaying the distorted image of an ROS imaging system with a less distorted image of an LED print bar imaging system results in a transferred image with noticeable distortion since corresponding black and color pixels are unevenly spatially separated along the scan line.
Known systems compensate for such scan line non-linearity differences electronically using a variable frequency pixel clock (sometimes called a scanning clock). The pixel clock produces a pulse train (i.e., a pixel clock signal) that is used to turn the light beam emitted by the light source of the ROS imaging system on and off at each pixel location along the aim line. Accordingly, varying the clock frequency and thereby the timing of individual pulses in the pulse train serves to control pixel placement along the scan line. The shape of the non-linearity signature varies, however, from one ROS imaging system to another ROS imaging system. Thus, each system is typically individually measured and programmed using complex measurements and computer programs.
As set forth in U.S. Pat. No. 6,178,031, known electro-photographic systems provide for such measurements and programming with various electro-photographic machines. Known methods used to generate the desired pixel clock modulation generally rely upon the electro-photographic machine to perform the computational functions to generate a correction. Such computations may be significant as the corrections may be in the form of a fifth order polynomial.
Some electro-photographic machines do not include the internal communication and processing capability to support such complicated corrections of scan line non-linearity. Nonetheless, the reduction of scan line non-linearity differences between imaging stations in such machines is still desired.
Accordingly, reduction of the distortion resulting from the overlay of the outputs from two imaging systems that do not have identical scan line non-linearity errors is needed. Furthermore, the reduction of such distortion in machines with two imaging systems and reduced communication and processing capability is needed. Additionally, the reduction of overlay distortions using simple programming and adjustments is needed.